1. Field of the Invention
The invention relates generally to fluoropolymers and methods of polymerizing fluoroolefins in a supercritical fluid reaction medium.
2. Description of the Related Art
Polymerized fluoroolefins are conventionally manufactured via emulsion polymerization processes. These processes are conceptually straightforward; however, significant complications are associated with heat and/or mass transfer control in the reactor. From an operational standpoint, the process is very sensitive to heat and mass transfer conditions which, if improperly controlled, can potentially lead to temperature runaway and pressure aborts. Accordingly, the product has a limited range of molecular weight—typically lower than 300,000—with a wide molecular weight distribution that is indicated by a polydispersity index higher than 5. It is further problematic that the reaction product contains a large amount of gel and microgel. Gel formation may necessitate cleaning of the solids, in additional process steps with commensurate reduction of process efficiency. Due to the emulsifying medium, which is typically water, the wet polymer product must proceed through complex finishing processes that include filtration and drying.
The polydispersity index is a measure of the distribution of molecular weights. The polydispersity index is calculated as the weight-average molecular weight divided by the number average molecular weight. As polymer chains approach a uniform chain length, the polydispersity index approaches unity or a value of one. Typical polydispersity indexes vary based on the mechanism of polymerization and can be affected by a variety of reaction conditions. A low polydispersity index indicates a uniform polymer material and a reaction mechanism does not have a tendency to chain-terminate.
To address the foregoing problems with emulsified reaction systems, some efforts have been made to synthesize polymers from their monomeric reactants in supercritical media. Fluids existing at states above critical temperature and pressure are called supercritical fluids. Any fluid may exist and function as a supercritical fluid, provided the temperature and pressure conditions exceed the critical values.
Most of the supercritical polymerization efforts involve polymerization of fluoropolymers and acrylics. For example, U.S. Pat. No. 6,340,722 issued to Lee et al. reports the production of polymethacrylate in supercritical media.
For example, liquid or supercritical CO2 is used in a reaction media with tetrafluoroethylene (TFE) as reported in U.S. Pat. Nos. 5,618,894; 5,674,957; 5,939,501; 5,939,502; and 5,981,673 issued to DeSimone et al. The process conditions exist over a wide range of temperatures and pressures without regard to consequent effects on polymer molecular weight and polymer morphology. While the reactions proceed fairly well for TFE, copolymers of VF2 are typically gummy. Japanese Patents 45,003,390 and 46,015,511 describe polymerization of TFE in CO2 media with use of 60Co γ-rays to initiate polymerization, as opposed to using a chemical-type free radical initiator. The radiation-initiated TFE polymerization reaction may be carried out at temperatures between −70° C. and +80° C.
Other fluoropolymers have been made in CO2 media, for example, as reported in U.S. Pat. No. 5,496,901 issued to DeSimone. A 1,1-dihydroperfluorooctyl acrylate material is polymerized and co-polymerized using aazobisisobutyronitrile (AIBN) as a free radical initiator. In addition, low molecular weight amorphous fluoroolefins are polymerized in the presence of C4F9I and UV light. All polymerizations are carried out in homogeneous solutions comprised of CO2, monomers, and resultant polymer. The process conditions exist over a wide range of temperatures and pressures without regard to consequent effects on polymer molecular weight and polymer morphology.
U.S. Pat. No. 5,496,901 describes polymerization of water-insoluble polymers in aqueous phase in the presence of CO2. Different monomers are polymerized using different initiators in the presence and in the absence of fluoro-surfactant with emphasis on preserving a ratio where TFE:CO2 is 50:50. The monomer is polymerized using K2S2O8. However, polymerization of water-insoluble polymers in aqueous media in the presence of CO2 results in a dispersion of resin in the aqueous media, which circumstance is ultimately similar to the conventional emulsion or suspension polymerization and provides no apparent advantage over the art.
U.S. Pat. No. 5,739,223 issued to DeSimone describes a method of making fluoropolymers using a two-stage reaction process with each stage conducted at a different temperature. The process conditions exist over a wide range of temperatures and pressures without regard to consequent effects on polymer molecular weight and polymer morphology.
U.S. Pat. No. 6,051,682 issued to Debrabander et al. proposes a continuous polymerization system to replace batch polymerization using CO2 as the polymerization media. In the proposed process, polymerization is carried in a continuously stirred tank reactor and the resultant polymer is collected in downstream filters. All examples pertain to TFE/PPVE with no teaching about VF2 polymerization. Moreover, no information is reported or claimed regarding molecular weight or molecular weight distribution of resultant polymers. The process conditions exist over a wide range of temperatures and pressures without regard to consequent effects on polymer molecular weight and polymer morphology.
WO0190206 to DeSimone et al. describes production of fluoropolymers with a multimodal molecular weight distribution. There is no teaching of how to make high molecular weight polymer with a unimodal molecular weight distribution. U.S. Patent Application 2002/0040118 A1 to DeSimone describes similar production of fluoropolymers with a multimodal molecular weight distribution. The polymer has a low average molecular weight typically lower than 200,000, a multimodal molecular weight distribution, and non-controllable morphology. The process utilizes very few initiators, which incidentally do not work particularly well as indicated in part by a monomer conversion of less than 20%.
U.S. Pat. Nos. 5,674,957 and 5,618,894 issued to DeSimone et al. disclose the use perfluorinated free radical initiators, mainly hexafluoropropylene (HFPO) dimer, in making fluoropolymers. Monomer may be dissolved in CFC-113, to produce polymers in CO2 media with stable (fluorinated) chain ends. The polymerization yields and average molecular weights of the resultant polymers are very low, except for the case when TFE is used as a co-monomer. When VF2 is used as co-monomer, a gummy polymer of very low molecular weight is obtained with yields less than 10%.
U.S. Pat. No. 6,103,844 issued to Brothers describes the use of dialkyl(2,2′-azobisisobutyrate) as an initiator in polymerization of TFE and also of VF2 in CO2 media. Other azo compounds, such as 2,2′-azobisisobutyronitrile(AIBN) or 2,2′-azobis(2,2-dimethylpentanitrile) are generally not effective initiators for fluoropolymers. These azo compounds are solids at ambient conditions; however, no information about their states under polymerization conditions is provided.
WO0146275 to Brown et al. describes a process of making oligomers and telomers of fluoromonomers in CO2. The fluoromonomers are principally TFE. The reaction occurs in the presence of a chain transfer agent, such as HBr and/or HCl. The initiator for telomerization is principally HFPO dimer, which is an ineffective initiator for VF2 polymerization.
Some patents describe the synthesis of polyvinylidine difluoride (PVDF) in a supercritical medium, for example, as in U.S. Pat. Nos. 5,496,901; 5,688,879; 5,739,223; 5,863,612; 5,922,833 issued to DeSimone. Analysis of prior processes for VF2 polymerization with use of supercritical CO2 reveals the following problems:                Pressure. Use of very high pressure results from attempts to synthesize polymers above or near the resultant polymer's cloud point in supercritical carbon dioxide. The required pressures are sometimes commercially impracticable, e.g., as in the case of a 45,000 psi process or a 10,000 psi process.        Monomer concentration. VF2 monomer loading is low—typically less than only a few percent of the supercritical fluid medium. With a low monomer loading in supercritical carbon dioxide at excessively high pressures, the chances for interactions of monomer-oligomer, monomer-monomer and monomer-initiator are significantly lowered by the dominating population of carbon dioxide molecules in the system. There is premature termination of free radicals.        Ratio of monomer to solvent. Because of the pressure and monomer concentration issues discussed immediately above, the relative concentration ratio between the fluoromonomer and the solvent is kept low, which in turn affects the product quality as well as the process efficiency, as is also explained above.        Supercritical solvent effects. There is a failure to understand and implement the use of reagents as cosolvents at supercritical reactions conditions, especially to investigate the possible effects of CO2 and VF2 as cosolvents to solubilize polymer reaction product and initiator when VF2 is a reagent. Therefore, the processes are operated more strictly as a precipitation polymerization from the beginning to the end of the reaction.        Initiator choices. There is a very narrow selection among initiator choices, and the initiators do not work particularly well. There is initiator involvement in reaction mechanism with consequent chain branching and chain termination effects leading to less desirable products. Selections do not include use of free radical initiators that are more common and less expensive. The choice of initiator is not based on reaction kinetics or mechanisms.        Polymer weight. Low molecular weight polymers result from these processes. The average molecular weight of product polymer is substantially lower than that of the conventional emulsion polymerization process.        Use of a stabilizer chemical or a fluoro-surfactant. These materials are unable to sustain continuing chain growth in CO2—VF2 systems. There may be formation of undesirable foam. The art commonly uses stabilizers and/or fluoro-surfactants for polymer chain growth as well as for product particle morphology.        Gummy product. There is formation of gels and microgels, such that the reactions do not directly result in gel-free free-flowing powder and additional process steps are required to harvest clean solids. The existence of gel or microgel is an indirect proof of the negative effect of melt processing or solution processing of product polymer. Further, it also provides an indirect proof for entrapment of inhomogeneous species inside the polymeric matrix/phase, e.g., solvent initiator, its fragmented derivatives, unreacted monomer, and interpenetrating network of oligomers and low weight fluoropolymers. Therefore, the product is impure and inferior not only from the material property standpoint, but also from the further processing aspect.        Product morphology controls. The references do not describe a control strategy to benefit product morphology.        Molecular weight controls. Besides the low molecular weight polymers, there is poor control over polymer molecular weight distribution. The processes are not selectively controllable to produce polymer with a desired molecular weight.        Inferior polydisperse product. The molecular weight distribution is multimodal, proving that the degree of polymerization is never under control. The resultant product is, therefore, significantly inferior.        Inability to achieve high molecular weight polymer. The references do not describe how to make polymer with high or medium molecular weight. The molecular weights, even in the best possible cases, are substantially lower than those of the conventional emulsion polymerization products. Thus, the polymer products are inferior to hose produced by emulsion processes.        Low process yields. The polymerization yield is low and the monomer conversion is also low, thus making the process operate in a high recycle mode and worsening the process economics.        